Such a optical disk drive is known from published patent application WO 2005/034100 A2. WO 2005/034100 A2 discloses a method of determining the setting of a spherical aberration correction to be applied to an optical beam in an apparatus for reading data from and/or writing data onto an optical data carrier, especially a BluRay disk or a HD-DVD disk. An incident beam is generated and focused onto the optical data carrier so as to produce a reflected beam. A spherical aberration correction is applied to the incident beam. A difference signal, which results from a difference between the intensities of two substantially symmetrical portions of the reflected beam, for example the radial push pull signal (PP) or the radial wobble signal, is measured during a relative movement between the incident beam and the optical data carrier. A specific setting (VO) of the spherical aberration correction, which maximizes the amplitude of the difference signal, is determined. Such a measurement will be referred to as a one-dimensional (1D) measurement as the value of one characteristic—e.g., a radial error signal—is measured as a function of a plurality of values of one parameter—i.e., collimator position or, equivalently, spherical aberration correction—. A problem of this prior art is that the quality of the obtained spherical aberration correction, although resulting in a large amplitude of the difference signal, is not always optimal for reading from and/or writing data on the disk.
Another such optical disk drive is described in the yet unpublished U.S. patent application Ser. No. 11/755,552 of the same applicant. U.S. patent application Ser. No. 11/755,552 describes an optical reading/writing apparatus having an optical head that includes a collimator and an objective lens. For focusing control, a start-up procedure is executed to generate a first start-up S-curve. A boundary is then set according to the start-up S-curve. After executing focusing on and tracking on, a plurality of position combinations of the collimator and the objective lens are selected for focusing calibration, thereby obtaining respective focusing error signals. By comparing the focusing error signals with the boundary, whether the position combinations of the selected collimator and the objective lens are valid can be determined. One of the valid position combinations with the greatest image-quality value is then selected to read/write the optical disc in the subsequent reading/writing procedure. Such as measurement will be referred to as a two-dimensional (2D) measurement as the value of one characteristic—i.e., image-quality—as a function of a plurality of combinations of two parameters—i.e., collimator position and objective lens position, or equivalently, spherical aberration correction and focus offset—, in which both parameters are varied. A problem of this prior art is that the quality of the selected combination of objective lens position (for focus offset) and collimator position (for spherical aberration correction) is not always optimal. When applied to a large number of optical disks in a large number of optical drives, it was found that the greatest image-quality based on the measurement of one parameter, e.g., the wobble amplitude, did not always correspond to the greatest image-quality based on measurements of another parameter, e.g., data jitter. This could result in, e.g., a selected combination with a good data jitter allowing to retrieve the written data well, but with a poor wobble amplitude, resulting in a poor recovery of the position along the track. This could result in, e.g., a selected combination with a good data jitter allowing to retrieve the written data well, but with a poor radial error signal, resulting in non-stable tracking performance. It was also found that it is sometimes very difficult to determine the image quality, as the measurements are sometimes difficult to analyse, e.g., difficult to fit with a function. It was also found that the procedure can take a considerable time, as the procedure requires to measure a relatively large number of objective lens positions for a relatively large number of collimator positions. Although the variation of the objective lens position can be done relatively fast, the variation of the collimator position takes a more significant amount of time. Hence, it would be advantageous to have a procedure which requires no, or only a limited number of, collimator position variations. It would also be advantageous to have a procedure which generally only needs to vary one position, preferably the position of the objective lens.